Duplex stainless steel alloys have a microstructure consisting of about 45 to 65% austenite and 35 to 55% ferrite. The two-phase structure prevents grain growth and the fine grain size thus contributes to the very high strength of duplex stainless steel alloys. Compared to austenitic stainless steel wire products, duplex stainless steel wire products are known to have excellent corrosion resistance and high mechanical strength in combination with good fatigue properties, and are therefore suitable for spring applications in for example seawater applications.
The production route for duplex stainless steel wire products starts with wire rod, annealed or non-annealed, with a diameter of 5.5-12 mm. The wire rod is pickled in hydrochloric acid to remove the oxide scale and a carrier, normally hydrated sodium, is coated onto the surface of the wire rod to facilitate drawing. The wire rod is then cold drawn in several reduction steps to the final wire dimension. An intermediate annealing step is usually necessary to reach final dimensions below 2.5 mm. The final wire product obtained can be used in for example spring applications where excellent corrosion resistance is necessary.
However, for certain applications, where the end product is subjected to severe high cycle fatigue and high dynamic stresses, such as springs for injection pumps, it is desirable to improve the fatigue properties of duplex stainless steel wire products. A well-known production route for conventional austenitic stainless steel and precipitation hardenable stainless steel wire products with improved fatigue properties includes a peeling step, which gives an end product with better surface quality and therefore less initiation points for cracks and consequently improved fatigue resistance. The production route for peeled austenitic stainless steel wire products includes, apart from the above mentioned steps used to produce duplex stainless steel wire products, a peeling step using a shaving die before drawing to final dimension, and an extra pickling step after peeling, including salt bath furnace, white pickling, hydrochloric acid and application of carrier coating. Because of the special properties of duplex stainless steel wire, such as its high mechanical strength and its combination of austenite and ferrite, it has however turned out difficult to adjust the peeling process such that it is possible to include in the production route for duplex stainless steel wire. The ferrite phase tends to stick on the shaving die while the austenite phase is tough and hard, which result in a high tool wear and poor chip breakage. Even in annealed condition, when the grain size of stainless steel alloys is usually larger, the grain size of duplex stainless steel alloys is small, which in combination with the chemical composition creates a very hard material. Thus, the properties of duplex stainless steel alloys make it difficult to peel duplex stainless steel wire rod.